Formation of cadmium chlorides via cadmium dissolution in chloride melts

Article abstract of DOI:10.1134/S0020168506010158

The kinetics and mechanism of the formation of complex Cd(I) ions via the reaction of metallic cadmium with Cd²⁺ ions in the Cd ⁰-CdCl₂-ZnCl₂-NH₄Cl system are studied spectroscopically. The formation of Cd₂²⁺ and Cd₂³⁺ is evidenced by absorption bands around 270 and 335 nm, respectively, in the electronic spectrum of the melt. The anode current efficiency is determined for cadmium electrorefining in a chloride melt. A mechanism is proposed for the anodic dissolution of cadmium at different current densities and process temperatures. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506010158

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 1, pp. 75–80. © Pleiades Publishing, Inc., 2006.
Original Russian Text © V.F. Kozin, A.A. Omel’chuk, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 1, pp. 77–82.
Formation of Cadmium Chlorides via Cadmium Dissolution
in Chloride Melts
V. F. Kozin and A. A. Omel’chuk
Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine,
pr. Akademika Palladina 32/34, Kiev, 03142 Ukraine
e-mail: omelchuk@ionc.kar.net
Received June 2, 2005
Abstract—The kinetics and mechanism of the formation of complex Cd(I) ions via the reaction of metallic cad-
mium with Cd2+ ions in the Cd0–CdCl2–ZnCl2–NH4Cl system are studied spectroscopically. The formation of
Cd22+ and Cd32+ is evidenced by absorption bands around 270 and 335 nm, respectively, in the electronic spec-
trum of the melt. The anode current efficiency is determined for cadmium electrorefining in a chloride melt. A
mechanism is proposed for the anodic dissolution of cadmium at different current densities and process tem-
peratures.
DOI: 10.1134/S0020168506010158
INTRODUCTION
At present, new refining methods are being devel-
oped in order to enhance the purification efficiency,
reduce the number of consecutive steps, and increase
the number of elements that can be removed in a single
process. One such method is electrolysis in molten
electrolytes [18]. Electrode processes in molten elec-
trolytes involve subvalent cadmium compounds [19].
For this reason, detailed information about the state of
cadmium ions in electrode layers is crucial for the abil-
ity to control such processes.
High-purity cadmium is one of the starting mate-
rials for the synthesis of II–VI compound semi-
conductors [1, 2] and CdTe–CdS, Cd1 – xZnxTe, and
CdxHg1 − x − yZnyTe solid solutions [3–5], which find
application in opto- and microelectronics. In addition,
high-purity cadmium is used in the fabrication of light-
emitting diodes [6], solar cells [7, 8], and detectors
based on the wide-gap semiconductors CdTe and
CdZnTe for radiation monitoring at nuclear power
plants [9]. CdTe/CdS p–n junctions and CdTe are used
in high-efficiency solar energy conversion and storage
applications [10, 11]. Ultrapure cadmium possesses a
large capture cross section for thermal neutrons and is
used to fabricate neutron-absorbing control rods for
nuclear reactors [12]. The physicochemical properties
of cadmium are attractive for nuclear power engineer-
ing applications, in particular for recovering and refin-
ing spent nuclear fuel in molten salt systems [13, 14].
Chemical equilibria in the systems Cd0–CdHal2
(Hal = Cl–, Br–, I–) have been studied extensively (see,
e.g., [20, 21]). Metal dissolution was shown to be
accompanied by changes in the anion component of the
salt and process temperature [20, 21]. In particular, the
equilibrium Cd+ concentration is 21.0 mol % in the
Cd0–CdCl2 system at 800°C, 20 mol % in the Cd0–
CdBr2 system at 700°C, and 15 mol % in the Cd0–CdI2
system at 700°C [17].
Taking into account Cd+ dimerization, the formation
The steady growth in the world market for high-
purity cadmium in recent years entails ever more strin-
gent requirements on cadmium purity and a need for a
significant cadmium production scale-up.
of subvalent cadmium compounds, and the presence of
Cd+, Cd2+, Cd22+ , Cd23+ , and finely dispersed cadmium
metal in the Cd0–CdHal2 systems, it is of interest to
monitor the anode current efficiency in the process of
cadmium dissolution.
High-purity cadmium can be prepared using a vari-
ety of methods and approaches, of which the most
widespread are electrolysis with the use of solid elec-
trodes [15], distillation followed by zone melting [16],
and zone refining in flowing hydrogen [17]. In most
instances, the purity of the resulting cadmium does not
meet semiconductor industry’s requirements. Only by
combining several refining processes can high-purity
cadmium be obtained.
Molten cadmium is known to react with cadmium
halides to form subvalent cadmium compounds [20–
28] according to the scheme
Cd0 + CdHal2
2CdHal,
(1)
where Hal = Cl–, Br–, or I–.
75

76
KOZIN, OMEL’CHUK
position in which a cadmium plate 2 cm2 in area had
1.4
1.0
0.6
0.2
been immersed. The light beam was passed along the
surface of the cadmium plate. The absorbance of the
melt in the cuvette was taken into account using a solu-
tion of the same composition in a reference cuvette. To
reduce the electrolysis temperature, cadmium chloride
was dissolved in the 45 wt % ZnCl2 + 55 wt % NH4Cl
eutectic [29], which melts at 180°C.
RESULTS AND DISCUSSION
Cadmium is a transition metal with the 4d105s2 outer
shell configuration. It has a tendency to form metal–
metal bonds stable in melts [24]. In earlier studies [25,
26], pulse radiolysis in aqueous solutions made it pos-
sible to identify a monovalent cadmium compound,
which had a strong absorption band at 295 nm.
In molten salts, the equilibrium of reaction (1) in the
Cd0–CdCl2 system is shifted to the right [22, 27, 28]. To
identify the intermediate compounds in the reaction
system under consideration, it is reasonable, in our
opinion, to use spectrophotometry.
250
300
350
400
Wavelength, nm
Figure 1 illustrates the time variation of the intensity
of absorption bands in the electronic spectra of Cd0–
CdCl2–ZnCl2–NH4Cl melts. The lowermost spectrum
was recorded 1 min after the cadmium plate had been
immersed in the melt, and the other spectra were taken
at 2-min intervals.
As seen in Fig. 1, the introduction of a cadmium plate
into the melt gives rise to a strong absorption peaked near
270 nm. Owing to the fast rate of reaction (1) and the
shift of equilibrium to the right, the Cd2+ concentration
in the melts studied was 5.0 mmol/cm3.
Fig. 1. Time variation of the intensity of absorption bands in
0
the electronic spectra of Cd –CdCl –ZnCl –NH Cl melts.
2
2
4
In this paper, we describe the formation of cadmium
subchlorides via the reaction of cadmium with Cd2+-
containing melts.
EXPERIMENTAL
In our preparations, we used extrapure-grade salts,
which were purified further and carefully dehydrated.
Purified anhydrous CdCl2 was prepared by passing
hydrogen chloride through molten cadmium at 650–
750°C. Chemical analysis for impurities showed that
the resultant cadmium chloride contained Al 5 × 10–5,
Bi 2.6 × 10–4, Fe 3.3 × 10–4, In 1.3 × 10–4, Co 2.0 × 10−4,
Mn 7.4 × 10–4, Cu 2.3 × 10–5, Ni 1.4 × 10–5, Pb 1.7 ×
10−5, Sb 2.8 × 10–4, Tl 1.5 × 10–4, and Zn 2.0 ×
10−4 wt %.
As shown earlier using pulse radiolysis [25], the Cd+
intermediate is a highly reactive species which tends to
dimerize:
Cd+ + Cd+
Cd22+ .
(3)
The formation of the Cd22+ dimer, a more stable spe-
cies, is responsible for the strong absorption band
around 270 nm (Fig. 1). The shift of equilibrium to the
right increases the lifetime of the Cd22+ ion, in accor-
dance with the results reported by Borresen et al. [30].
The absorption spectra in Fig. 1 show a shoulder at
about 335 nm, which may be due to the reaction
Prior to electrolysis, the electrolyte was held in con-
tact with extrapure-grade (99.9999%) cadmium metal
at 600°C for 3 h in order to saturate it with subchlo-
rides:
CdCl2 + Cd0 = Cd2Cl2.
(2)
Cd+ + Cd2+ = Cd32+ .
(4)
The resultant melt was dark pink.
To elucidate the mechanism and follow the kinetics
of the formation of intermediate cadmium compounds,
we used spectrophotometry (Specord UV–VIS instru-
ment). The melt was enclosed in a leak-tight quartz
cuvette with plane-parallel windows and a 1-cm optical
path. The kinetics of Cd+ formation were evaluated by
According to Ershov [31], in concentrated aqueous
solutions Cd+ reacts with Cd2+ to form Cd32+ clusters.
Note that, in the spectral range corresponding to the
formation of Cd22+ and Cd32+ cations, the absorbance of
monitoring the absorbance of the melt of known com- the reaction mixture as a function of Cdmx+ content
INORGANIC MATERIALS Vol. 42 No. 1 2006

FORMATION OF CADMIUM CHLORIDES VIA CADMIUM DISSOLUTION
77
obeys the Bouguer–Lambert–Beer law (Fig. 2). This
finding suggests that, measuring the absorbance as a
function of time, one can spectroscopically follow the
1.4
1.2
1.0
0.8
(a)
kinetics of Cd22+ and Cd32+ formation according to
schemes (3) and (4).
Let A0 be the absorbance of the reaction mixture at
the instant in time when equilibrium (3) set in, Aτ be the
absorbance at time τ, and A∞ be the absorbance after a
time sufficiently long for the complete transition of
cadmium ions from one oxidation state to another. The
difference Aτ – A∞ is then proportional to the Cd+ con-
centration at time τ, and A0 – A∞ is proportional to that
at the beginning of the measurements. Under the
assumption that equilibrium (3) is described by a first-
order rate law, the equilibrium concentration of monov-
alent cadmium compounds in the reaction system can
be represented by the equation
5
6
7
8
Cd22+ concentration × 105, g-ion/l
(b)
0.6
0.4
0.2
(Aτ – A∞) = (A0 – A∞)exp(–kτ)
(5)
or
log(Aτ – A∞) = log(A0 – A∞) – kτ/2.303,
(6)
20
30
40
50
where k is the rate constant for the formation of the cor-
responding compound. The plot of log(Aτ – A∞) ver-
sus τ will then give a straight line with a slope k/2.303.
Analysis of the present data indicates that, in the
case of Cd22+ and Cd32+ formation, the variation in the
absorbance of the reaction mixture is well represented
by Eq. (6) (Fig. 3).
The rate constants for the formation of the Cd22+ and
Cd32+ complex cations evaluated from the slope of
lines 1 and 2 in Fig. 3 are 4.22 × 10–3 and 3.50 × 10–3 s,
respectively. Thus, our experimental data provide clear
evidence that cadmium reacts with molten CdCl2 to
form Cd22+ and Cd32+ .
Cd23+ concentration × 105, g-ion/l
0
Fig. 2. Absorbance of Cd –CdCl –ZnCl –NH Cl melts as a
function of (a) Cd22+ and (b) Cd32+ concentrations upon the
2
2
4
formation of the corresponding complex cations.
log(A0 – A∞)
1
–1.0
–0.8
–0.6
–0.4
–0.2
2
Figure 4a shows the anode current efficiency as a
function of current density j for the anodic cadmium
dissolution. The current efficiency of cadmium dissolu-
tion rises from 100% at j = 0.05 A/cm2 to 124% at j =
1.0 A/cm2, which is associated with side reactions and
the participation of Cd+ ions in electrode processes. In
the range of current densities 0.05–0.2 A/cm2, the cur-
rent efficiency is close to the value predicted theoreti-
cally for Cd2+ compounds:
0
2
4
6
8
Time, min
0
Fig. 3. Semilog plots of absorbance vs. time for Cd –
CdCl –ZnCl –NH Cl melts upon the formation of
2
2
4
(1) Cd22+ and (2) Cd32+ complex cations.
ëd0 = Cd2+ + 2e–.
(7)
addition to the formation of stable Cd2+ ions, to the for-
mation of subvalent cadmium compounds:
The observed tendency for the current efficiency to
rise with increasing current density is due to the forma-
tion of subvalent cadmium compounds. The same is
evidenced by the observed changes in the color of the
Cd0 = Cd+ + e–.
(8)
melt. Therefore, increasing the current density leads, in The stability of these compounds is enhanced owing to
INORGANIC MATERIALS Vol. 42 No. 1 2006

78
KOZIN, OMEL’CHUK
chemical oxidation,
Cd22+ – e = 2Cd2+
η, %
130
(9)
(10)
(11)
(a)
followed by the chemical reaction
Cd22+
120
110
ëd+ + Cd+
Cd0 + ëd2+.
Increasing the current density favors the reaction
Cd32+ .
Cd+ + Cd2+
100
0
0.2
0.4
0.6
0.8
j, A/cm2
Convincing evidence for the formation of the para-
magnetic dimeric ion Cd32+ was provided in an ESR
study by Eachus and Symons [32]. The Cd32+ ion is sta-
bilized owing to the bonding between two metal ions
via an electron in a σ-orbital. As pointed out by Ale-
ksandrov et al. [33], an insignificant axial anisotropy of
the g-factor and an only slight deviation of g from
2.00 point to a small contribution of the p and d orbitals
to the σ molecular orbital.
(b)
120
110
100
As the temperature is raised, the current efficiency
of the anodic dissolution of cadmium increases. As
shown above, increasing the temperature drives the
equilibrium in reaction (1) toward the formation of Cd+
ions, in accordance with earlier results [22, 27, 28]. Fig-
ure 4b shows a typical plot of the anode current effi-
ciency versus temperature for chloride electrolytes at
j = 0.2 A/cm2.
580 600 620 640 660 680
t, °C
Fig. 4. Current efficiency as a function of (a) current density
at 600°C and (b) electrolysis temperature at a current den-
2
sity of 0.2 A/cm for the anodic dissolution of cadmium.
the dimerization reaction (3), which ensures the forma-
tion of an unshared pair of 5s electrons [20]. The
dimeric ions forming by reaction (3) undergo electro-
The high current efficiency of the anodic dissolution
of cadmium indicates that the electrolyte contains cad-
mium ions in different oxidation states. We evaluated
the average oxidation state of cadmium from the reduc-
tion in the amount of cadmium on the anode under dif-
ferent electrolysis conditions using the formula n =
A/(26.8η), where η is the current yield of the anodic
dissolution of cadmium, g/(A h), and A is the atomic
weight of cadmium. The results are presented in the
table. The average oxidation state of the cadmium
resulting from anodic dissolution is seen to depend on
the electrolysis conditions and to be below 2. At j =
0.05 A/cm2, the oxidation state of cadmium is 2+,
which confirms that the electrode process involves two
electrons. With increasing current density, the oxida-
tion state of cadmium decreases, which attests to the
formation of subvalent cadmium compounds by reac-
tions (3), (4), and (10).
Characteristics of cadmium dissolution in a cadmium chlo-
ride electrolyte at 600°C
Oxidation state
Current
Amount
Anode current
of cadmium
resulting from
density, of Cd dissolved efficiency,
A/cm2 at the anode, g
g/(A h)
anodic dissolution
0.05
0.1
0.2
0.4
0.6
0.8
1.0
1.479
3.020
2.095
2.137
2.179
2.306
2.409
2.515
2.599
2.00
1.962
1.925
1.819
1.741
1.667
1.614
6.160
13.032
20.430
28.430
36.724
CONCLUSIONS
The kinetics and mechanism of the formation of
subvalent cadmium complexes in the Cd0–CdCl2–
ZnCl2–NH4Cl system were studied spectroscopically.
INORGANIC MATERIALS Vol. 42 No. 1 2006

FORMATION OF CADMIUM CHLORIDES VIA CADMIUM DISSOLUTION
79
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